Coatings that provide low reflectivity and/or a high percent transmission over a broad wavelength range of light are desirable in many applications including solar cells, windows, and the like. Light transmission through material causes the wavelength of the light to change, a process known as refraction, while the frequency remains unchanged thus changing the speed of light in the material. Antireflective (AR) coatings are typically applied to the surface of a transparent substrate to reduce reflectance of visible light from the article and improve transmission of such light through the substrate.
Sol-gel based antireflective (AR) coatings using alkyltrialkoxysilane binders having low refractive index are described in U.S. Ser. No. 13/273,007, filed Oct. 13, 2011, the disclosure of which is hereby incorporated herein by reference. For example, the '007 application discloses coating a substrate with a sol-formulation comprising an alkyltrialkoxysilane-based binder having the formula (I):
where R1, R2, and R3 are the same or different and each represents an alkyl group containing 1 to 20 carbon atoms, an aryl group containing 6 to 20 carbon atoms, or an aralkyl group containing 7 to 20 carbon atoms, wherein R4 represents an alkyl group containing 1 to 20 carbon atoms, an aryl group containing 6 to 20 carbon atoms, an aralkyl group containing 7 to 20 carbon atoms, or a fluoro-modified alkyl group containing 1 to 20 carbon atoms, and silica based nanoparticles, wherein a mass ratio of the alkyltrialkoxysilane-based binder to the silica based nanoparticles is between 0.1:1 to 20:1. The sol-gel formulation also includes an alcohol containing solvent and an acid or base containing catalyst, in addition to the alkyltrialkoxysilane-based binder. After a glass substrate is coated with the sol gel, the coated glass substrate is annealed.
The term “binder” as used herein refers to a component used to bind together one or more types of materials in mixtures. The principal properties of a binder are adhesion and/or cohesion. The term “sol-formulation” as used herein is a chemical solution comprising at least a silane-inclusive and/or silane-based binder and silica inclusive and/or silica-based nanoparticles. The term “sol-gel process” as used herein is a process where a wet formulation (the “sol”) is dried to form a gel coating having both liquid and solid characteristics. The gel coating is then heat treated to form a solid material. This technique is valuable for the development of coatings because it is easy to implement and provides films of substantially uniform composition and thickness.
The AR coatings in the '007 application thus relate to a wet chemical film deposition process using a specific sol-formulation including a alkyltrialkoxysilane-based binder and silica based nanoparticles to produce porous anti-reflective coatings with a low refractive index (e.g., lower than glass). The sol-formulation in the '007 application may be prepared by mixing the alkyltrialkoxysilane-based binder, silica based nanoparticles, an acid or base containing catalyst, water, and a solvent system. The sol-formulation may be formed by at least one of a hydrolysis and polycondensation reaction. The sol-formulation may be stirred at room temperature or at an elevated temperature (e.g., 50-60 degrees Celsius) until the sol-formulation is substantially in equilibrium (e.g., for a period of 24 hours). The sol-formulation may then be cooled.
A transparent glass substrate is coated with the sol-formulation. The coating on the substrate is then dried to form a gel. A gel is a coating that has both liquid and solid characteristics and may exhibit an organized material structure. During the drying, solvent of the sol-formulation is evaporated and further bonds between the components, or precursor molecules, may be formed. The drying may be performed by exposing the coating on the substrate to the atmosphere at room temperature or a heated environment. The gel is then annealed to form the porous coating. E.g., the annealing temperature may be in the range of 300-1,000 degrees C.
Unfortunately, it has been found that the AR coating of the '007 application (described above) is lacking with respect to durability. Thus, it will be appreciated that there exists a need in the art for a more durable AR coating.
In certain example embodiments of this invention, there is provided a method of making a coated article including an anti-reflection coating on a glass substrate, the method comprising: forming a first sol formulation comprising a colloidal solution including a tri-alkoxysilane based binder; forming a second sol formulation comprising a colloidal solution including a tetra-alkoxysilane based binder; blending the first and second sol formulations to form a coating sol formulation; coating at least a portion of said coating sol formulation onto the glass substrate to form a coating; and heating said glass substrate and said coating. It has surprisingly and unexpectedly been found that this technique results in a more durable coating than the coating of the '007 application.